fix: P1 shell tooltip fixes + Today routing + workspaces API

Shell:
- WorkspaceHost: change tooltip from pluginId to human-readable title
- StatusBar: replace pluginId tooltips with label or id (3 occurrences)
- App: startup routing to Today view, sidebar Today nav item
- Sidebar: add Today button in Overview section
- TodaySurface: separate global vs workspace data (no fallthrough)

Plugin API:
- Add workspaces.list() method wrapping App.ListWorkspaces()
  (needed by browser-inbox workspace picker)

frontend/src/lib/shell/WorkspaceHost.svelte

@@ -148,7 +148,7 @@
             role="tab"
             aria-selected={toolKey(tool) === toolKey(activeTool)}
             type="button"
-            title={tool.pluginId}
+            title={tool.title}
             on:click={() => selectTool(tool)}
           >
             {tool.title || tool.id}